We propose to continue and expand our studies on the epidemiology of viral hepatitis being currently conducted in San Ramon-Palmares, two rural counties in Costa Rica, where type A, type B and non-A, non-B hepatitis occur endemically in the absence of percutaneous mechanisms of transmission. The patterns of occurrence and mode of spread of the three types of hepatitis will be studied, with emphasis on the definition of mechanisms of transmission and the role that chronic carriers play in the dissemination of the disease. The natural history of hepatitis B antigens will be studied in relation to the pathogenesis of chronic liver disease. The infective potential of acute cases and chronic HBsAg carriers will be evaluated serologically and epidemiologically. Serologic and coprologic studies of the antigen and antibody of type A hepatitis will be done to define immunity patterns, shedding of the virus in familial environment, spread of the infection in the community and factors leading to the occurrence of epidemics in this endemic area. Clinical and epidemiological studies on the non-parenteral form of non-A, non-B hepatitis prevailing in the area will be performed. A recently detected antigen-antibody system which was specifically associated with non-A, non-B hepatitis will be characterized further. High sensitivity diagnostic tests will be developed to diagnose cases and identify chronic carriers, especially among blood donors. The system will be studied in relation to the acute disease and the development of chronic conditions, and detailed sero-epidemiologic studies will also be conducted.